The present invention relates to gelled fluids, and methods and apparatus for using liquefied petroleum gas in subterranean operations. More particularly, the present invention relates to servicing fluids that comprise gelled liquefied petroleum gas or servicing fluids that comprise a conventional gelled hydrocarbon fluid with liquefied petroleum gas and methods of using such servicing fluids in subterranean formations.
Servicing fluids are used in a variety of operations and treatments performed in oil and gas wells. Such operations and treatments include, but are not limited to, production stimulation operations, such as fracturing, and well completion operations, such as gravel packing.
An example of a production stimulation operation using a servicing fluid having particles suspended therein is hydraulic fracturing. That is, a type of servicing fluid, referred to in the art as a fracturing fluid, is pumped through a well bore into a portion of a subterranean zone to be stimulated at a rate and pressure such that fractures are formed or enhanced in a desired subterranean zone. The fracturing fluid is generally an ungelled liquid or gas, a gelled liquid, emulsion, or foam that may comprise a particulate material often referred to as proppant. When used, proppant is deposited in the fracture and functions, inter alia, to hold the fracture open while maintaining conductive channels through which such produced fluids can flow upon completion of the fracturing treatment and release of the attendant hydraulic pressure.
An example of a well completion operation using a servicing fluid having particles suspended therein is gravel packing. Gravel packing treatments are used, inter alia, to reduce the migration of unconsolidated formation particulates into the well bore. In gravel packing operations, particulates, referred to in the art as gravel, are carried to a well bore in a subterranean producing zone by a servicing fluid known as a carrier fluid. That is, the particulates are suspended in a carrier fluid, which may be viscosified, and the carrier fluid is pumped into a well bore in which the gravel pack is to be placed. As the particulates are placed in the zone, the carrier fluid leaks off into the subterranean zone and/or is returned to the surface. The resultant gravel pack acts as a filter to separate formation solids from produced fluids while permitting the produced fluids to flow into and through the well bore. While screenless gravel packing operations are becoming more common, traditional gravel pack operations involve placing a gravel pack screen in the well bore and packing the surrounding annulus between the screen and the well bore with gravel that is sized to prevent the passage of formation particulates through the pack with produced fluids, wherein the well bore may be oriented from vertical to horizontal and extend from tens of feet to thousands of feet. When installing the gravel pack, the gravel is carried to the formation in the form of a slurry by mixing the gravel with a carrier fluid. Such gravel packs may be used to stabilize a formation while causing minimal impairment to well productivity. The gravel, inter alia, acts to prevent the particulates from occluding the screen or migrating with the produced fluids, and the screen, inter alia, acts to prevent the gravel from entering the well bore.
In some situations the processes of hydraulic fracturing and gravel packing are combined into a single treatment to provide stimulated production and an annular gravel pack to prevent formation sand production. Such treatments are often referred to as “frac pack” operations. In some cases the treatments are completed with a gravel pack screen assembly in place with the hydraulic fracturing treatment being pumped through the annular space between the casing and screen. In this situation the hydraulic fracturing treatment ends in a screen out condition creating an annular gravel pack between the screen and casing. This allows both the hydraulic fracturing treatment and gravel pack to be placed in a single operation. In other cases the fracturing treatment may be performed prior to installing the screen and placing a gravel pack.
In carrying out hydraulic fracturing, frac packing, and gravel packing, fluid recovery oftentimes may be critical. Foamed fluids have been developed in part to provide enhanced fluid recovery through energization by a compressed gas phase. They also reduce the total amount of liquid used, typically by a factor of about four. Such foamed fluids have included various surfactants, known as foaming and foam stabilizing agents, for facilitating the foaming and stabilization of the foam produced when a gas is mixed with the servicing fluid. Thus, foamed fluids may be thought of as media in which a relatively large volume of gas is dispersed in a relatively small volume of liquid, usually with the aid of a surfactant that reduces the surface tension of the fluids. The most commonly used gases for foamed fracture fluids are nitrogen, carbon dioxide, and combinations of the two. Foamed servicing fluids may be preferred over conventional servicing fluids because they generally provide superior fluid recovery as well as excellent fluid loss control without forming a substantial filter cake. Enhanced fluid recovery is provided by the expansion of the gas in the foam when the pressure is released after the stimulation and/or treatment. This promotes flow of residual servicing fluid liquid back into the well, thus aiding in cleanup of the servicing fluid once the subterranean operation is complete.
The use of conventional aqueous-based servicing fluids in subterranean operations may present disadvantages. For instance, the high capillary pressures associated with the use of an aqueous system may restrict the flow of produced gaseous hydrocarbons such as methane. Capillary pressures of several thousand psi may result in low permeability formations when water is introduced, wherein the high pressure differential needed to initiate the fluid flow may result in extended fluid recovery times, long term losses in the relative permeability to gas and long term loss of effective fracture half length. Furthermore, the use of water in under-saturated reservoirs also may reduce permeability and associated gas flow through a permanent increase in the water saturation of the reservoir.
The use of a carbon dioxide miscible hydrocarbon servicing fluid may overcome these limitations through achievement of a miscible drive mechanism where produced methane is used to displace the hydrocarbon fracturing fluid from the formation. To facilitate this process, more volatile hydrocarbon blends may be used in place of traditional hydrocarbon servicing fluids such as diesel fuel. For example, carbon dioxide may be added to the hydrocarbon-based servicing fluids, inter alia, to increase the efficiency by which methane can displace it and provide increased energy for fluid recovery and thus its rate of recovery from the subterranean formation. However, increasing concentrations of dissolved carbon dioxide in the liquid hydrocarbon make it progressively more difficult to gel with phosphate ester and alkylphosphonic acid ester gel systems. As a result there is a limit to the concentration of carbon dioxide that may be present in such servicing fluids. For instance, if too high a concentration of carbon dioxide is present, the servicing fluid may not have a viscosity sufficient to carry the needed quantity of particulates to a desired location within a well bore, to adequately control fluid leak off, and to generate the desired fracture geometry. In some instances a pure carbon dioxide fluid may be injected as a spearhead fluid to help energize the reservoir and create a miscible solvent layer ahead of the fracturing fluid to assist in fluid recovery.
Moreover, as a fracture or a gravel pack is created, a portion of the liquid contained in the servicing fluid may leak off into the formation and/or may create a filter cake comprising deposited viscosifier on the walls of the fracture, well bore, or the formation. In addition, conventional water-based servicing fluids may comprise polysaccharide-based polymers, which may serve as a food source for bacteria. Therefore, when deposited in the subterranean formation, such polysaccharide-based polymers may produce a bio-mass that may reduce formation permeability. While formation of a filter cake during pumping may be desirable to help control fluid leak off, it is not desirable for the filter cake to be permanent since it may restrict subsequent gas and liquid flow.
High viscosity gelled hydrocarbon liquids have heretofore been utilized in treating subterranean formations penetrated by well bores, in hydraulic fracturing stimulation treatments. In such treatments, a high viscosity gelled liquid hydrocarbon fracturing fluid having particulate proppant material, e.g., sand, suspended therein is pumped through a well bore into a subterranean formation to be stimulated at a rate and pressure such that one or more fractures are formed and extended in the formation. The suspended proppant material is deposited in the fractures when the gelled hydrocarbon fracturing fluid is broken and returned to the surface. The proppant material functions to prevent the formed fractures from closing whereby conductive channels remain through which produced fluids can readily flow to the well bore.
Polyvalent metal salts of orthophosphoric acid esters have heretofore been utilized as gelling agents for forming high viscosity gelled liquid hydrocarbon fracturing fluids. Such gelled liquid hydrocarbon fracturing fluids have included proppant material and breakers for causing the fracturing fluids to break into relatively thin fluids whereby the proppant material is deposited in formed fractures and the fracturing fluid is produced back. Descriptions of such heretofore utilized high viscosity gelled liquid hydrocarbon fracturing fluids and methods of their use are set forth in U.S. Pat. Nos. 4,622,155 and 5,846,915, the entire disclosures of which are incorporated herein by reference. The gelled liquid hydrocarbon fracturing fluids described in the above patents utilize ferric iron or aluminum polyvalent metal salts of phosphoric acid esters as gelling agents and delayed breakers such as hard burned magnesium oxide.
While there are several benefits that can be achieved in using hydrocarbon based fracturing fluids in reservoirs where there is a significant amount of water sensitivity, there have also been safety concerns with the use of these fluids due to their flammability and in some cases high vapor pressure. Much of the safety exposure occurs around the on site blending units that are traditionally used to add all of the chemical additives to viscosify the hydrocarbon fluid and also to add the proppant agents to the gelled fluid as it is being pumped down hole. Conventional blenders incorporate proppant metering screws that have a rate of rotation that is matched with the downhole pumping rate so that precise amounts of proppant can be added per unit volume of the fluid. The mixing of the proppant in the hydrocarbon usually occurs in an open top blending tub. The open blending tub presents several potential concerns that have resulted in limited use of hydrocarbon based fracturing fluids in many locations around the world. Some of these concerns are: 1) high vapor pressure of the fracturing fluid which can result in hydrocarbon fumes accumulating in the top of the bender tub creating a potentially dangerous condition as it is dispersed into the atmosphere, and 2) the need for continuous blending units which require control of the inflow fluid rate as well as the proppant rate to ensure that the fluid level in the blending tub remains constant. Sudden shut downs or sudden increases in flow rates can cause the tub level to fluctuate and in extreme cases result in fluid over flowing causing serious spills of highly flammable and environmentally damaging fluids. While the use of gas blankets, in which a layer of dense carbon dioxide or other inert gas such a nitrogen gas is continuously maintained on the top of the blender tank and automatic tub level control systems have helped to minimize the level of exposure and the associated risk, potential for high risk exposure is still present.
Similarly, problems may be encountered as a result of the use of particular gelling agents in the high viscosity gelled liquid hydrocarbon fracturing fluids, i.e., the polyvalent metal salt of a phosphoric acid ester. That is, in recent years plugging of refinery towers, which process oil produced from formations fractured with gelled liquid hydrocarbon fracturing fluids, has caused many expensive, unplanned shut-downs. The plugging material is high in phosphorus and has been attributed to the phosphate esters used as gelling agents. The phosphate esters contribute volatile phosphorus which condenses on distillation tower trays, causing plugging. The volatile phosphorus may also carry over the tops of the distillation towers causing contamination of the hydrocarbon products produced. This problem has been addressed in U.S. Pat. No. 6,511,944, the entire disclosure of which is incorporated herein by reference.
The gelling of liquid petroleum gas (“LPG”) comprising a mixture of varying amounts of methane, ethane, propane, butane and the like is disclosed in U.S. Pat. No. 7,341,103, the entire disclosure of which is incorporated herein by reference. In addition, U.S. Publication No. 20070204991 A1, the entire disclosure of which is incorporated by reference, provides a method for introducing proppant into an LPG fluid by pressuring the proppant in a pressure vessel with an inert gas and then metering that proppant into the LPG fluid utilizing a continuous blending unit to feed a high pressure pump which raises the pressure of the proppant containing slurry to a level sufficient to enter the subterranean formation. However, this method limits the proppant which can be delivered because of the specialized delivery apparatus required and potentially introduces an inert gas into the LPG fluid which can disrupt the pumping equipment. This method of proppant addition increases safety exposure of employees involved in the process of introducing the proppant into the LPG stream.